The present invention relates to improving hiding efficiency in pigmented paints using a phosphorus acid functionalized latex binder in combination with a water-soluble polymer containing sulfonic acid groups or salts thereof.
The efficacy of the TiO2 as a hiding pigment is reduced when TiO2 particles are allowed to come too close together on film formation and drying. It is known that the spacing of TiO2 and its concomitant hiding efficiency can be improved with the aid of emulsion polymer particles adsorbed to the TiO2 particle surface, as disclosed in US 2003/0018103. One of the problems observed with current adsorbing latex (pre-composite) technology, particularly latexes prepared using highly reactive phosphorus acid functional monomers such as phosphoethylmethacrylate (PEM), is formation of grit arising from the uncontrolled reaction of the reactive pre-composite with TiO2. Accordingly, a formulator must carefully mix the adsorptive latex with the pigment under controlled conditions to avoid flocculation, or decrease the level of reactive functional monomers, which decreases hiding efficiency. Additionally, the use of phosphorus acid monomers can be prohibitively expensive for some applications or can compromise viscosity stability of paint or dried film performance properties such as water sensitivity or scrub resistance. Furthermore, from a synthetic standpoint, a phosphorus acid monomer can adversely affect emulsion stability and seeding efficiency within emulsion polymerizations; moreover, this acidic monomer also lowers the pH of the process, thereby requiring subsequent neutralization, which lowers the potential volume solids of consequent latex. It would therefore be desirable to discover a way to decrease the concentration of phosphorus acid monomer in a pre-composite without compromising hiding efficiency, or alternatively to improve the hiding efficiency within a pre-composite without needing to increase the phosphorus acid monomer concentration.